The present invention relates to an improved process for purifying .epsilon.-caprolactam, (the starting material for nylon-6), obtained through Beckmann rearrangement of cyclohexanone oxime.
Such .epsilon.-caprolactam contains several impurities which must be removed in order to obtain a product suitable for polymerization. This impurity removal is effected in a number of processing steps, usually categorized into a pre-purification and final purification steps. The final purification is normally carried out by subjecting the .epsilon.-caprolactam containing impurities but already having a high degree of purity, such as about 99%, to distillation under reduced pressure.
According to German Offenlegungsschrift No. 2035859, dehydrated liquid crude .epsilon.-caprolactam can be distilled by employing three rectifiers and three evaporators under non-adiabatic conditions to obtain a purified .epsilon.-caprolactam product. The said rectifiers are carried out as thin layer rectification columns in order to minimise thermal degradation.
In practice, however, it has been observed that even greater .epsilon.-caprolactam purity is desirable in order to obtain better final polymerization products and to reduce capital costs in processing plants and equipment for handling .epsilon.-caprolactam. It has been observed that such a need exists for increasing the purity of caprolactam obtained by the rearrangement of cyclohexanone oxime by means of sulfuric acid or eleum.
The present invention satisfies this need by providing a distillation process for final purification of dehydrated but impure caprolactam (water content less than 1% by wt.) yielding very pure caprolactam and which requires less complicated and less expensive equipment.